The present invention relates to thermally insulated tubes, and, more particularly, the invention relates to tubing which is comprised of a seamless metal tube, preferably a drawn copper tube; a foil formed around that tube to provide a second, coaxial outer tube; and a spacer construction which includes a polymer foam, providing additionally a thermally insulating function as between the first-mentioned inner tube and the outer, tubular foil.
The German printed patent application No. 2,613,741 discloses a thermally insulated tube construction of the type referred to above which includes a metal tube, and a metal ribbon coaxially enveloping the tube and being spaced therefrom by a synthetic foam. In particular, a foam on the basis of polyisocyanurate is used which has been made flame resistant in a manner described in that printed application. Also, the metal ribbon is coated with a flame-resistant copolymer. The foil or metal ribbon formed into an outer tube serves as a kind of mold. Upon forming this tube, the edges of the foil are folded up, resulting in a radially outwardly extending and cemented tab which is subsequently folded down onto the surface of this tube.
This known tube is quite expensive to make, and the foil edges and tab are highly visible. Moreover, the tube just barely meets flame-resistant standards as, for example, laid down in the German Industry Standard DIN No. 4102, and then only in the beginning. Sooner or later, the flame-resistant additives diffuse more or less rapidly out of the foam, so that the actually provided protection lasts temporarily only. Moreover, the known flame-retarding additives which are included in the foam act frequently also as softener, thereby interfering with the spacing and support function of the insulation. The insulation foam becomes also thermally instable in some cases.